Exhaust gas purification systems using ceramic catalytic converters are well known as means for removing carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) contained in exhaust gases from automobile engines. Ceramic catalytic converters basically accommodate, for example, a honeycomb-shaped ceramic catalyst carrier (also referred to as “catalyst element”) inside a metal casing, in other words, a housing.
It is well known that there are ceramic catalytic converters of a variety of types, but usually a configuration is employed in which a gap between a casing and a catalyst carrier accommodated therein is filled with a holding or mounting material (e.g., in the form of a mat) generally obtained by combining inorganic fibers with a liquid or paste-like organic binder. As a result, the mounting material filling the gap holds the catalyst carrier and can prevent mechanical shocks caused by impacts, vibrations, and the like from being unintentionally applied to the catalyst carrier. Accordingly, no fracture or movement of the catalyst carrier occurs in catalytic converters of such a configuration. Therefore, the desired action can be implemented over a long period. Furthermore, the above-described mounting material has a thermally insulating function as well as a holding function.
When a pollution control element (e.g., a catalyst carrier) is loaded into a casing, a pressure insertion system is generally employed such that a mounting or holding material is wound about the outer periphery of the pollution control element and integrated therewith and the integrated body is thereafter inserted under a pressure into a cylindrical casing. Pollution control element holding materials of various types have been suggested to improve productivity of loading (also called “canning”) of the pollution control element in pressure insertion systems, increase cushioning ability (bulkiness) of the holding material, and prevent inorganic fibers constituting the holding material from being scattered in the air. See for example, Japanese Patent No. JP-A-2001-259438 claims, which suggests a catalytic converter comprising a catalyst carrier, a metallic shell (casing) covering the outer periphery of the catalyst carrier, and a holding and sealing material disposed between the catalyst carrier and the casing. In the case of this catalytic converter, the holding and sealing material, which corresponds to the holding material as referred to herein, comprises an inorganic fiber mat subjected to needle punching in the density of 50 to 3000 per 100 cm2 has a content of organic component largely over 0 and not more than 2 wt. %, and generates a surface pressure of 5 to 500 kPa when heated to a temperature of 300 to 1000° C. at a filling density of 0.15 to 0.45 g/cm3.
Furthermore, see for example Japanese Patent No. JP-A-2002-4848 claims, which suggests a catalyst carrier holding material used in a catalytic converter of a configuration similar to that described hereinabove, wherein a binder of 0.5 to 20 weight % of an organic binder or an inorganic binder is added and attached to a mat-like material with a mat-like arrangement of inorganic fibers, the filling density after assembling is adjusted so as to be within a range of 0.1 to 0.6 g/cm3, and when the ratio of solids in the binder added and applied to the mat-like material is evaluated for three equal sections (upper section, medium section, and lower section) in the thickness direction, the ratio of solids in the binder in the upper section and lower section is higher than that in the medium section.
Furthermore, see for example Japanese Patent No. JP-A-2002-206421 claims, which suggests a holding sealing material for a catalytic converter comprising ceramic fibers assembled into a mat-like configuration as constituent elements and disposed in a gap between a catalyst carrier and a metal shell covering the outer periphery of the catalyst carrier, wherein a peak-and-valley structure composed of an inorganic substance is provided on the outer surface of the ceramic fibers. In this holding sealing material, the peak-and-valley structure composed of an inorganic substance preferably comprises metal oxide particles having a mean particle size of 50 to 1,000 nm.